Removal of Pesticides From Water by Nanofiltration

Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 1, No.1, October 2012 _________________________________________________________________________________

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Removal of Pesticides From Water by Nanofiltration

RIUNGU N. J., Dept of Biomechanical and Environmental Engineering, J. K. U. A. T., Kenya HESAMPOUR M., Dept of chemical Technology, Lappeenranta University of Technology, Finland PIHLAJAMAKI A., Dept of chemical Technology, Lappeenranta University of Technology, Finland MANTTARI M., Dept of chemical Technology, Lappeenranta University of Technology, Finland HOME P. G., Dept of Biomechanical and Environmental Engineering, J. K. U. A. T., Kenya NDEGWA G. M., Dept of Biomechanical and Environmental Engineering, J. K. U. A. T., Kenya ABSTARCT Agricultural activities form the backborne of Kenya´s economy. Inorder to control crop losses, pesticides are used and in the recent past, more of the pesticides have been used to increase production. However, the effect of pesticides on the environment is very complex as undesirable transfers occur continually among different environmental sections. This eventually leads to contamination of drinking water source especially for rivers and lakes located near active agriculture practices especially flower farms around Lake Naivasha in Kenya where the lake is a major source of livelihood to sorrounding environment. Poisoning of lake water by pesticides caused fish deaths whose impact was felt in the whole republic of Kenya. This paper studied the application of nanofiltration membranes in the removal of atrazine in aqueous solution. Due to extensive use of atrazine to control weeds and the adverse environmental effect associated with it, it was selected as the subject of the study. Separation was done using a laboratory scale crossflow filtration units that operated in total recycle mode to ensure even concentration of atrazine in the feed solution. Concentration of atrazine in aqueous solution was analyzed using high performance liquid chromatography (HPLC). Four nanofiltration membranes NF90, NTR7250, and NF270 were tested for their respective performance to separate atrazine. Effect of feed solution pH, concentration and feed pressure were investigated. Of concern also was the effect of humic substances and titanium dioxide catalyst on rejection efficiency of the the membranes. pH and feed pressure had significant influence on rejection of atrazine while initial feed concentration had little influence on rejection efficiency. The presence of HA led to improved atrazine rejection efficiency but led to flux decline on all membrane tested while TiO2 led to high rejection efficiency and insignificant flux decline. Of all four membranes, NF90 showed the best performance in retention of atrazine in water while NTR7250 showed the least.

1.0 INTRODUCTION In recent years various international and local regulations have become stricter concerning the amounts of pollutants in wastewaters and the quality of the treated effluents discharged into the aquatic environment. This is mainly because the pollutants are known or suspected to cause harmful ecological effects. Widespread concerns are being raised due to the increasing number of cases when such contaminants are detected in surface water bodies, and due to their potential to affect the development, reproduction and health of wildlife, livestock and even humans. Most of contaminants found in any aquatic environment mainly constitutes of organic compounds. Organic matter found in water spans a wide spectrum, with

molecular weights ranging from several thousands to less than a hundred g/mol. Most compounds on the upper end of this spectrum are of natural origin, and they are commonly known as natural organic matter (NOM). Trace organics are generally located at the lower end of the organic compound spectrum. The trace organics include pesticides, trihalomethanes (THMs), polychlorinated biphenols (PCBs) and polyaromatic hydrocarbons (PAHs) and are commonly reffered as persistent polar pollutants (POPs) due to their persistence in the environment. POPs have been identified as an increasing problem in our drinking water supplies. Such substances can enter the waters supply from various sources and are not effectively removed by

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conventional water treatment processes. Pesticides have been classified as POPs due to their resistance to natural degradation processes, and hence their ability to remain in the environment for long periods of time. By their very nature, they are designed to be toxic and kill unwanted organisms but can attack non target organisms and as a result cause serious environmental damage. Due to the extensive use of pesticides in industry and agriculture, many water sources are contaminated with pesticides especially wastewater from agriculture farms and pesticides formulating or manufacturing plants (Shaalan et al.,2007). Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) has been a widely used herbicide to control certain annual broadleaf weeds and grasses throughout the world over the last 50 years. It has been found in groundwater at concentrations exceeding the ground and drinking water limit of 0.1 µg L-1 of the European Union (Spliid and Koppen, 1998). It is less expensive and persists longer in the soil than alternative herbicides, with a lifetime of days even years in the environment. It as been known to cause serious effects to humans, animals and the aquatic environment. Conventional water treatment processes, specifically coagulation–flocculation, sedimentation and conventional filtration, are ineffective in removing pesticides from drinking water.(Plakas et al., 2006) Traditional plants are far from efficient and offer removal levels that rarely exceed 10–20% for atrazine and 40% for simazine (Zhang et al., 2004). Disinfection and water softening, however, may lead to pesticide transformation and formation of disinfection byproducts (USEPA, 2001). Removal of pesticides for the production of drinking water can be conducted by activated carbon filtration (Herrera et al., 2006, Acero et al., 2009) but it is an expensive procedure that requires frequent regeneration. This is because organic micropollutants such as pesticides may be present at the mg/l level whereas NOM concentrations may be 10,000 times higher, hence the adsorption columns have to be regenerated rapidly because the column capacity is mainly used for NOM adsorption instead of pesticides adsorption (Bruggen et al., 2003). In parallel, some chemical treatments have been applied for the reclamation and reuse of different wastewaters and surface waters containing pesticides, by using several

oxidants like ozone, hydrogen peroxide, UV radiation, and their combinations (advanced oxidation processes) (Ormad et al., 2008, Acero et al., 2009). Ozone is known to produce a variety of aldehydes (Nghiem, 2005). Over the past few years, nanofiltration membranes have been studied as potentially useful means of pesticide removal considering the fact that the molecular weights (MWt) of most pesticides are more than 200 Da (Kamrin et al., 1997, Plakas et al., 2006). Nanofiltration has been successfully applied in drinking water treatment plant in Mery-sur-Oise, France (Cyna et al., 2002) Leiduin (Bonne et al.,2000) and Heemskerk (Hofman et al.,1997) Holland as well as Saffron Walden in England. However, there is still a long list of pesticides in guidelines for drinking water by World Health Organization, (2004) but there is lack of adequate data for their effective separation using membranes. Therefore, there is still room for the investigation of the feasibility of using membrane technology to completely remove atrazine from water. The main objective of this study was to investigate the removal of atrazine by NF membranes. The effect of operating parameters of the feed solution on the rejection efficiency for atrazine; pH, concentration and feed pressure were investigated. Of concern too was the effect of humic substances and titanium dioxide catalyst on the rejection efficiency and operation of the membranes. In the filtration experiment, three NF membranes were tested namely; NF90, NTR7250 and NF270. Analysis of the atrazine concentration in the samples was done by HPLC. 2.0 EXPERIMENTAL PROCEDURE The experiment was conducted to investigate the retention of atrazine by three NF membranes. The effect of solution pH on retention of atrazine by the membranes was assessed. Three different pH values for the feed solution were used namely; pH 4, pH 7.2 and pH 9. 2.1 Membranes Past research as shown that membrane processes, such as reverse osmosis (RO) and nanofiltration

(NF) are considered promising candidates for the removal of low molecular weight organic compounds of environmental concern, like

pesticides (Plakas and Kalabelas, 2009, Shaalan et al., 2007). The molecular weight of most pesticides are in the range of 200-400 g/mol.



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Three NF membranes were investigated in this study. The characteristics of the membrane used are

shown on Table 1.

Table 2-1; Characteristics of membranes used in the study NF270 NF90 NTR7250

Manufacturer Dow (Filmtec) Dow (Filmtec) Nitto denko, Japan MWCO 200a 200a 300-450b

Zeta potential (mV) -21.6c -24.9c -6e

Contact angle 28±2c 62±2c Naf

Membrane pore size (nm) 0.71±0.14c 0.55±0.13c 0.65a

NaCl retention 66.4c 99.5c 50e

Membrane material Polyamide d Polyamide d Polyvinyl alcohol e

pH range 3-10d 3-10d 2-9e

Maximum temperature 45d 45d 40e

a- As stated by manufacturer b- Verliefde, et al, 2005 c- Plakas and Kabelas, 2008 d- Ahmad et al, 2008 e- Nymston et al., 1995 f- na (not available)

2.2 Herbicide The herbicides atrazine which has had a significant share of the herbicide market and is detected with great frequency in drinking water sources, was selected for the experiments. Herbicide analytical standards was purchased from Sigma–Aldrich with

atrazine (purity 97.4%). The molecular structure and physicochemical properties of the tested herbicide is presented in Table 2. The herbicide is hydrophobic (log Kow > 2), moderately soluble in water and therefore weakly polar compound (Plakas and Kalabelas, 2008).

Table 2-2; Properties of herbicide used in the study (W. S. S. A., 1994) Atrazine Chemical structure

Molecular formula C8H14ClN5

Molecular weight (g/mol) 215.69 Molecular size (nm)a 0.788 Log Kow

2.68 Aqueous solubility (mg/L) 33

a- Bruggen et al., 1998



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Solution preparation A standard stock solution was prepared for atrazine in high-performance liquid chromatography (HPLC)-grade methanol and stored at 4 0C with concentration 100 mg/L. The feed atrazine solutions were prepared from pure water by dilution of stock solutions at a level of 10 mg/L and 20 mg/L. The experiments were carried with feed solutions at pH 4, pH 7, pH 10 and natural pH of atrazine solution which is pH 8.3. The pH of the atrazine feed solution, was adjusted to different initial pH by adding 1M NaOH or 37% (w/w) HCl (Fisher Scientific, Pittsburg, PA). The pH measurement was conducted using pH meter (Mettler Toledo Delta 320 pH Meter). Methanol was used for preparation of stock solutions but the cosolvent effect was not considered in this paper. To test for retention of 10 ppm atrazine in presence of humic substances and TiO2, humic acid was used to simulate the organic matter found in natural water where humic acid sodium salts was supplied by Sigma–Aldrich company. and 10 ppm titanium dioxide solution (TiO2), (C380 from TIPE Company, China) with primary particle size 6-8 nm. Since humic substances concentrations in natural waters usually fall in the range 2–40mgL−1 (Jones et al., 1998) the solution was prepared with ultra-pure water and with a concentration of 10mg L−1 humic acid. The HA was obtained in powder form and used without further purification. 2.4 Filtration setup The filtration was carried out in a laboratory scale cross flow filtration unit that operated in total re-cycle mode where permeate and retentate were returned to the feed tank. A schematic presentation of the unit is shown in the Fig.1 below. The model solution in the feed tank was pumped to flat sheet membrane module by a centrifugal pump. The unit consisted of three membrane modules arranged in parallel. The required pressure and flow velocity were achieved by controlling the power of the pump and the back pressure valve after the membrane module. The effective surface area of each membrane module was 20.48 cm2. The temperature was adjusted by re-circulating a cold water bath around the feed tank.

Fig.2.1; Schematic diagram of cross flow filtration unit

2.5 Filtration procedure Filtration was carried out using the cross flow setup described in section Fig.2.1. The feed tank capacity 5 L was filled with the feed solution. Prior to commencement of experiment, there was measurement of pure water flux. The filtration experiment was carried out at pressure of 6 bar and 12 bar respectively and a velocity of 2.5 m/s and temperature of 25oC. Temperature was maintained constant by re-circulating a cold water bath around the feed tank. The filtration protocol involves a sequence of the following steps; 1) At first, the membrane was rinsed with tap water for several minutes and afterwards it was fitted on the modules, then membrane compaction with pure water under pressure of 15 bar, for 4 hours, to ensure that the removal of preservatives from the new membrane coupon was complete (Plakas et al. 2008). Compaction is crucial in every membrane filtration protocol as it may change both the active layer and its support, thus affecting the flux and the rejection properties of the membrane.



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To eliminate this impact, membranes are often subjected to at a higher pressure (here 15 bar) than the operating pressure (12 bar (maximum for this study)) to ensure flux stability during experiments (Schafer et al., 2005) 2) Measurement of the pure water flux at 6 bar, then, stabilized water flux at different operating pressures was obtained and membrane permeability values (Lp) were determined from the slope of flux against pressure graph. (3) Filtration of10 ppm atriazine at 6 bar and 12 bar, each for 3.5 hours respectively at natural pH (8.3) of the solution, (4) Filtration of 20 ppm atriazine at 6 bar and 12 bar, each for 3.5 hours respectively at natural pH of the solution, (6) Filtration of 10 ppm atrazine solution at pH of 4, 7 and 10 respectively each for a duration of 3.5 hours. (7) Filtration of 10 ppm atrazine solution in presence of 10 mg/L TiO2 catalyst and 10 mg/L HA. solution respectively. After each experiment, the membranes were rinsed with pure water at same conditions as was the filtration process and pure water flux measurements made.

Permeate from the bottom of the membrane modules was collected on 30 minute interval and its weight was measured. The cumulative weight was converted to cumulative permeate volume (Vp), and the permeate flux (Jw for purewater, or Jv for the atrazine solutions) was obtained by means of equation 1:

.......................................................Eqn 1 Where is the accumulated permeate flux during the time difference and A is the membrane area. At the same time,samples were collected for analysis of atrazine concentration by use of HPLC. 2.6 Analytical method Concentration of atrazine in feed and permeate was analysed using high performance liquid chromatography (HPLC) by Perkin Elmer (USA). Isocratic eluent: 20 mM ammonium hydroxide-methanol mixture (50:50, v/v). flow rate: 0.15 ml/min. Column: Phenomenex C18 reversed phase (100 mm x 21 mm, 3 µm particles. Percentage of rejection was obtained by use of equation 2. The effectiveness of a membrane is measured on how much of the feed material is retained during

operation. This is termed the removal efficiency and is calculated using the following equation:

........................Eqn.2

where R is the observed retention, cf the concentration of the feed and cp the concentration of the permeate 3.0 RESULTS AND DISCUSSION 3.1 Membrane permeability The Fig. 3.1 shows membrane permeate flux at increasing pressures.

Fig.3.1; Membrane permeate flux Vs pressure at vel. 2.5 m/s, temp. 25oC

The slope of the sraight line joining the points gives the membrane permeability. Membrane permeability is a measure of rate of permeate flow across the membrane per unit area per unit pressure. The highest permeability was recorded by NTR7250 with 11.58 L/m².hr.bar while NF270 was second with a flow of 10.506 L/m².hr.bar. NF90 took the last position with a permeability of 5.89 L/m².hr.bar. The differences obtained could have been caused by membrane material differences as well as membrane pore size difference. It has been reported in literature that , NF270 has average pore size of 0.71 nm, NF90 has average pore size 0.55 nm while NTR7250 has average pore size of 0.65 nm (Plakas and Kalabelas, 2008, Zhu et al., 2003).

y = 5.883x + 1.848R² = 0.989

y = 11.58x - 3.106R² = 0.995

y = 10.50x - 1.528R² = 0.992

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3.2 MEMBRANE RETENTION PERFORMANCE

3.2.1 Influence of pH, concentration and pressure on membrane retention

The retention performance of atrazine by NF90, NTR7250, NF270 at different pressure, concentration and pH is presented in Fig.3.2.

Fig 3.2; Membrane retention performance at flow velocity 2.5 m/s, temp. 250C

From these figures, it is obvious that the retention of atrazine tend to be better when the pressure was increased from 6 bar to 12 bar. It can be seen that NF90 produced the best retention performance for the operating pressure and feed concentration tested, at more than 95% retention for atrazine. The performance of NF270 was the second highest of all three membranes tested while NTR7250 showed lower retention than NF270 when both were operated at the same pressure and feed concentration. Higher retention was observed at higher pressure due to the increased water flux that caused dilution of permeate from the membranes. Similar findings were reported by Armad et al., (2008) during filtration of dimethoate and atrazine using four nanofiltration membranes at different pressures.

In this experiment, the removal efficiency did not vary greatly, regardless of the initial triazine concentration. The retention results with triazine solutions are in agreement with observations made by other researchers (Zhang et al., 2004, Plakas et al., 2009) in that herbicide concentration does not significantly affect their retention. The fact that the filtration of lower feed concentrations leads to a slight reduction of atrazine retention could be attributed to the amount of atrazine adsorbed on the selected membranes. In practical terms, the consequence of this result is that different stages of a nanofiltration plant have the same efficiency level so far as atrazine is concerned.

The influence of the pH on the rejection is shown in Fig. 3.3.

Fig. 3.3; Membranes retention performance at various pH conditions; Temp. 250C, crossflow velocity 2.5 m/s, pressure of 12 bar.

NF90 membrane showed almost consistent retention for all tested conditions while the NF270 and NTR7250 showed varying % retention at different solution conditions. The rejection was the highest at pH 7; at pHs 4 and 10, the rejections were consistently lower. This was attributed to ion adsorption: at higher pH, OH− ions can adsorb on membrane surface, resulting in an increase of the membrane charge. Polar components such as pesticides have a lower rejection when the membrane charge increases, because they are dipoles which can have a preferential orientation towards the membrane in the sense that the side of

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the dipole with a charge opposite to the membrane charge is the closest to the membrane. In this way, the preferential orientation results in an increased attraction, an increased permeation and thus a lower rejection. At lower pH, the same effect might occur with H+. The NF90 and NF270 have the same MWCO but NF90 recorded the highest retention of over 95%. Usually, the MWCO is defined as the MW of a solute that was rejectedat 90 percent (Bruggen et al., 1999). NF90 was rather chemical-resistant as it showed somewhat consistent performance regardless of the solution’s pH. There was only a drop of about 2% of rejection performance for NF90 compared to the obvious increase or reduction of rejection performance shown by the rest of the nanofiltration membranes tested. The NF90 and NF270 membranes are slightly different although with the same polyamide thin-film composite. NF270 has a very thin semi-aromatic piperazine-based polyamide active layer while NF90 consists of a fully aromatic polyamide active layer (Ngiem et al., 2004) while NTR7250 is made of a combination of poly vinyl alcohol and piperazine trimesamide (Nymstron et al., 1995). This slight difference of membrane structures is one of reasons that NF90 showed superior rejection characteristics compared to the NF270 membranes tested at the experimental conditions.

Puasa, 2006 reported that polyamide thin-film composite membranes have charge characteristics that influence the separation capabilities, which can be altered by the solution’s pH and it was reported that the isoelectric point of polyamide membrane is generally between 4 and 5. According to Nymstron et al., 1995 the isoelectric point of poly vinyl alcohol is between 3 and 4. The occurrence of an isoelectric point means that at lower pH than the isoelectric point, the membrane is positively charged and vice-versa. Hence, in the case of polymeric membranes, surface membrane charge is typically negative at high pH values, it increases as the pH decreases and switches to positive values at low pH’s (Bandini and Mazzoni, 2005).

3.2.2 Retention of herbicides in presence of organic matter and TiO2

In the the membrane separation experiments in which humic substances and TiO2 were mixed together with herbicides, the final feed solutions were first prepared and placed in a foil-covered container (to prevent herbicide degradation by exposure to light) and stirred for 24 h, after which they were assumed to be at equilibrium- a protocol

used by earlier researchers (Devitt et al., 1998). The nanofiltration experiments show significant influence of humic acids acting on the retention of atrazine as shown in Fig. 3.4.

Fig. 3.4; Membranes retention performance in presence of HA and TiO2; pressure 12 bar, vel. 2.5 m/s, temp. 250C

This is attributed to the formation of complexes between humic acids and herbicide, which enhance the rejection by steric exclusion. According to Chiou et al. (1986) hydrophobic humic substances of high molecular weight are not very soluble in water and display a stronger interaction with non-ionic complexes such as triazines (e.g. atrazine). This interaction between herbicides and humic substances can be attributed to the large number of functional groups characterizing the structure of humic materials.

The effect of humic substances on atrazine adsorption and retention is in agreement with other studies; the explanation is that a low energy bond between humic substances and triazines is established by physisorption which results in an increased steric exclusion of the humic substance-atrazine pseudocomplex (Kulikova et al., 2002, Plakas et al., 2006). Moreover, the density of the complex negative charge increases due to the primary negative charge of humic substances, while the adsorbability of the complex on the

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surface of the membrane is enhanced due to the hydrophobic nature of humic acid.

The nanofiltration experiments in which TiO2 was mixed with atrazine show significant effect on retention of atrazine by the membranes. This was caused by adsorption of atrazine on the surface of the catalyst forming large complexes which facilitated rejection by molecular sieving effect. TiO2 catalyst has been reported in many studies for its effectiveness in degradation of organic matter. It has been noted in previous studies that photocatalytic process mainly occurs on the catalyst surface and not in the bulk solution (Li et al., 2002) hence the first step is adsorption on catalyst surface. In presence of HA and catalyst all membranes recored removal efficiencies of over 80%.

3.3 PERMEATE FLUX PERFORMANCE

3.3.1 Influence of solution pH, feed pressure and and concentration.

Fig. 3.5 show the flux performance of the membranes for atrazine retention.

Fig. 3.5; Membranes flux performance at temp. 250C, velocity 2.5 m/s

Based on the figure, it was obvious that the increase in pressure had significant effect on permeate flux for the atrazine retention tests. All membranes tested experienced approximately double increment of permeate flux when the operating pressure was doubled from 6 bar to 12 bar. This shows that permeate flux increment corresponded linearly to the pressure applied to the solution. Meanwhile, concentration of feed had very little effect on the permeate flux as compared to operating pressure.

Thus, effect of concentration can be excluded from consideration when it comes to flux performance NF270 produced the highest permeate flux for all conditions tested. NTR7250 showed the second highest permeate flux out of the three membranes in all tested conditions NF90 showed the lowest permeate flux among the membranes.

Based on the published data, NF270 had average pore size of 0.71 nm, NF90 had average pore size 0.55 nm while NTR7250 had average pore size of 0.65 nm (Plakas et al., 2008, Zhu et al., 2003). Hence, the results obtained in this study agreed to the average pore size reported in the literature, that the pore size has an influence on membrane permeate flux. However, this also showed that while 0.55 nm average pore size for NF90 was sufficient to retain atrazine with high percentage of rejection, solute-membrane interaction factor was also important (Bellona et al., 2004, Kim et al., 2005) as NF270 showed better retention than the NTR7250 despite the fact that NF270 had a bigger pore size.

The effect of solution pH on permeate flux is as shown on Fig. 3.6.

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Fig.3.6; Membranes flux performance as a function of solution pH, at temp. 250C, vel. 2.5 m/s, pressure 12 bars.

In all the membranes tested, the permeate flux was considerably low at pH 4 and 10 while pH 7 recorded the highest permeate flux. This was probably caused by changes on membrane surface charge. It has been reported in several studies that below membranes isoelectric point, the negative surface charge of membrane reduces while above the point (high pH), the negative charge on membranes surface increases (Armad et al., 2008, Nymstron et al., 1995). Hence, at high pH values the membrane becomes more hydrophillic and vice versa at low pH values. Increase in membranes hydrophillicity causes more water to permeate through the membrane pores hence higher flux.

3.3.2 Influence of HA and TiO2 on permeate flux

The presence of HA in atrazine feed solution caused a decline in permeate flux while TiO2 showed a slight decline in flux as shown on Fig. 3.7.

Fig. 3.7; Membranes permeate flux performance in presence of HA and TiO2, at temp. 250C, crossflow velocity 2.5 m/s, pressure 12 bars

The permeate flux decline caused by HA was evident in all membranes tested. HA is hydrophobic and adsorbs on membrane surface. This adsorption leads to reduction in effective pore size causing reduction in permeability of the membrane. TiO2 do not adsorb on membrane surface. Instead, the organic matter adsorbs on its surface and in presence of light causes degradation of organic matter.

4.0 CONCLUSION

The performance of nanofiltration membrane to retain atrazine in aqueous solution was examined in this study. Three nanofiltration membranes, NF90, NF270 which have molecular weight cut-off of around 200 g/mol and NTR7250 with MWCO between 300 -450 g/mol were subjected to laboratory crossflow filtration unit and the effect of feed concentration, operating pressure pH HA and TiO2 on the permeate flux and rejection of atrazine was investigated. It was found that increasing the transmembrane pressure posed positive effect on atrazine rejection and permeate flux.

However, effect of feed concentration had little effect on the performance of the membranes tested. The pH of feed solution had influence on permeate performance and rejection efficiency of the membranes tested. The best rejection was achieved at pH7 and low at pH 4 and 10. Polar components such as pesticides have a lower rejection when the membrane charge increases, because they are

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dipoles which can have a preferential orientation towards the membrane in the sense that the side of the dipole with a charge opposite to the membrane charge is the closest to the membrane. In this way, the preferential orientation results in an increased attraction, an increased permeation and thus a lower rejection. At lower pH, the same effect might occur with H+. On the other hand, permeate flux increased with increasing pH. This was attributed to changes in the surface charge of the membrane.characteristics. Above the membranes iso electric point, the membrane becomes more hydrophillic hence more water permeates through the membrane and vice versa at low pH. In presence of humic substances, the rejection was found to increase while the flux declined which was attributed to adsorption of HA on the membrane surface thus narrowing down the membrane pores. TiO2 presence led to an increase in rejection and did not much affect the permeate flux.

NF90 showed the best retention performance while NF270 showed the highest permeate flux out of the three membranes tested. However, good retention quality should be the primary property in choosing the appropriate nanofiltration membrane for application in pesticides treatment from water. Therefore, despite its high permeate flux, NF270 is not suitable as the permeate quality was not good.

NF90 is deemed the more suitable nanofiltration membrane for atrazine retention from aqueous solution since, it showed the highest retention of atrazine coupled with considerably good permeate flux. Use of TiO2 was also found to be good as it led to increase in permeate quality while maintaining the flux of the membranes.

Acknowledgements

The authors would like to acknowledge the financial support of the CIMO-NSS scholarship funds. We wish to thank Helvi Tukia and Prof. Heli of LUT who helped in analysis of the samples.

References

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Removal of Pesticides From Water by Nanofiltration